This invention relates to a composition which is suitable as a rheology-influencing agent for the production of pigmented and non-pigmented coating media which are resistant to run-off, particularly for stoving systems. The invention also relates to the preparation of the composition and to coating media containing the latter. Requirements and regulations relating to environmental protection and workplace hygiene dictate that the amount of solvents in coating media should be significantly reduced. Development work in this field has been concentrated on raising the solids content of the coating medium (high solids). It has been possible to achieve this object by reducing the molecular weight of the resins. However, after application to vertical areas, to edges and in crevices, and particularly during the stoving operation, coating media of this type exhibit a pronounced run-off tendency ("run formation", "sagging"). Run-off can be impeded by converting a coating medium with quasi-Newtonian flow behaviour into one with plastic or pseudo-plastic flow behaviour.
Thus it is known, for example, that coating materials with a viscous structure can be used where thick film lacquer systems are desired or where low molecular weight, heat-curable lacquers of high solids content are used, which tend to be resistant to run-off. Numerous literature references exist which deal with the resistance to run-off of stoving lacquers, e.g. DE 2360019, DE-A-2359923, DE-A-2359929, DE-A-2751761, EP-A-0 192304 and EP-A-0 198519.
Common to all of these is the formation of a di- or polyurea compound by the reaction of primary, secondary, mono-, di- or polyamines or alcoholamines with mono-, di- or triisocyanates in a carrier resin which is added to the lacquer later.
The run-off stabilising effect of polyurea compounds is based on the formation of hydrogen bridges, the loose network of which stabilises the lacquer film on vertical support surfaces. The network is first destroyed due to shear during the spray application and is formed again (built up) on the substrate after application.
A prerequisite for this effect (resistance to run-off) is the presence of polyureas in particulate form. The particle size, which is produced by means of high stirring speeds using special stirrers and metered addition devices, is important in order to obtain a constant, uniform effect. Simple slow-running stirrer units, such as those which are used in the production of synthetic resins, are unsuitable for this purpose (EP-A-0 192 304). Another disadvantage is that not all the customary lacquer solvents can be used without the resistance to run-off being adversely affected. This applies in particular to polar solvents e.g. monohydric alcohols such as n-butanol (EP-A-0 192304).
One serious disadvantage which occurs on the use of the known polyureas is bloom formation in stoved lacquer films, particularly when run-off preventing agents are used in clear lacquers. Smooth film surfaces are frequently not obtained; instead surfaces are obtained with pinholes or with a microstructure, or matt surfaces are obtained. The result of this is reduced gloss and reduced brilliance.